Tough pick to be honest but Im gonna go with for me
Gunners at their best.
Pst....that album is from 1987, but I would agree with you if it came out in 1991. Don't feel bad, my first instinct was to go with Hysteria by Def Leppard, but that is also a 1987 album. I didn't realize how old I was getting.
Anyways, probably the best rock album I consider in the last 20 years would be RATM debut album RATM. I might be breaking the rules, but I consider them rock. It was a revolutionary and groundbreaking album for Rock n' Roll.
Other honorable mentions are:
Tool: Aenima. Another revolutionary and groundbreaking album.
Guns N' Roses: Use Your Illusion II. Need I say more?
RHCP: Blood Sugar Sex Magik. I don't know how you improve on greatness, but leaving Soul to Squeeze on the album would have made this album that much better.
White Stripes: Its a toss up between Get Behind Me Satan, White Blood Cells, and Elephant, but they deserved to be mentioned.
The Black Crowes: Amorica. By far their best album by a great rock band that infused 1990's rock with Southern Rock.
Pearl Jam: Vs. Ten is more influential, but I prefer Vs.
Soundgarden: Superunknown. Great band, great album, and Chris Cornell is probably the greatest rock voice in the past 20 years (and a pretty decent guitarist).
Neil Young: Harvest Moon. Probably not one of the greatest albums in the past 20 years, but a personal favorite.